The entire northern hemisphere is experiencing a sustained period of warming that is unprecedented in the past millennium, a study has found.

A review of a range of temperature records, from tree rings and ice cores to historical documents, has found that at no time since the 9th century have temperatures been so consistently high. The study, published in the journal Science, found that the late 20th century was the warmest period for the northern hemisphere since at least 800AD, eclipsing the well-known medieval warm period when vines were cultivated successfully in northern Europe and the Vikings exploited the ice-free seas to colonise Greenland.

Timothy Osborn and Keith Briffa, climate scientists from the University of East Anglia in Norwich, analysed 14 sets of temperature records from America, Europe and East Asia. Each record covered a relatively wide region, such as northern Sweden or the low countries of the Netherlands and Belgium, and extended back at least several centuries.

Ten of the 14 records were based on tree-ring data, which went back as far as 800AD, one measured ice cores from Greenland, one involved historical documents from Europe and one covered the chemical composition of sea shells on the east coast of the US. The final set of records came from China and Japan and used a variety of records, from ice cores to historical documents."Our results show that, during the late 20th century, warming affected the entire northern hemisphere and that at no point in the past 1,000 years has the northern hemisphere experienced the same widespread warming," Dr Osborn said....

"The key conclusion was that the 20th century stands out as having unusually widespread warmth, compared to all of the natural warming and cooling episodes during the past 1,200 years," Dr Osborn said.

Greenland's southern glaciers have accelerated their march to the Atlantic Ocean over the past decade and now contribute more to the global rise in sea levels than previously estimated, researchers say.

Those faster-moving glaciers, along with increased melting, could account for nearly 17 percent of the estimated one-tenth of an inch annual rise in global sea levels, or twice what was previously believed, said Eric Rignot of NASA's Jet Propulsion Laboratory in Pasadena, California.

An increase in surface air temperatures appears to be causing the glaciers to flow faster, albeit at the still-glacial pace of eight miles to nine miles a year at their fastest clip, and dump increased volumes of ice into the Atlantic.

That stepped-up flow accounted for about two-thirds of the net 54 cubic miles of ice Greenland lost in 2005. That compares with 22 cubic miles in 1996, Rignot said.

Rignot and his study co-author, Pannir Kanagaratnam of the University of Kansas, said their report is the first to include measurements of recent changes in glacier velocity in the estimates of how much ice most of Greenland is losing.

"The behavior of the glaciers that dump ice into the sea is the most important aspect of understanding how an ice sheet will evolve in a changing climate," Rignot said.

"It takes a long time to build and melt an ice sheet, but glaciers can react quickly to temperature changes."

Details of the study were being presented Thursday at the annual meeting of the American Association for the Advancement of Science. The study appears Friday in the journal Science.

The researchers believe warmer temperatures boost the amount of melt water that reaches where the glaciers flow over rock.

That extra water lubricates the rivers of ice and eases their downhill movement toward the Atlantic. They tracked the speeds of the glaciers from space, using satellite data collected between 1996 and 2005.

If warmer temperatures spread to northern Greenland, the glaciers there too should pick up their pace, Rignot and Kanagaratnam wrote.

The only way to stem the loss of ice would be for Greenland to receive increased amounts of snowfall, according to Julian Dowdeswell of the University of Cambridge, who wrote an accompanying article.

About Me

Been spending most of my time doing the chores that come with a permaculture homestead: a garden, food forest and chickens. My other time is spent at a telescope or doing a bit of freelance web design.